Metal parts joined with sintered powdered metal

ABSTRACT

Metal parts are joined together with sintered powdered metal by applying a mixture of powdered metal and an organic heat-fugitive binder to the parts at the locus of the joint of to be formed therebetween, assembling the parts in their desired joined configuration, and heating the assembly to volatilize or burn-off the binder and sinter the powdered metal.

United States Patent 91 Bergstrom et a1.

[ Feb. 13, 1973 METAL PARTS JOINED WITH SINTERED POWDERED METAL [75]Inventors: Theodore R. Bergstrom, Little Canada; Philip D. Takkunen,Woodbury Township, Washington County, both of Minn.

[73] Assignee: Minnesota Mining and Manufacturing Company, St. Paul,Minn.

[22] Filed: Sept. 21, 1970 [21] App1.No.: 73,891

[52] US. Cl. ..29ll82.2, 29/191.2, 29/192, 29/471.1, 75/208 R [51] Int.Cl. ..B22f 1/00 [58] Field of Search ..75/208 R; 29/182.2, 471.1,29/192,191.2

[56] References Cited UNITED STATES PATENTS 3,534,464 10/1970 Lallemant..29/471.1X

3,479,231 1 H1969 Mantel et a1 ..29/471.1 X

3,419,388 12/1968 Arutunian ..75/208 R 3,436,307 4/1969 Johnson et a1.29/l82.2 X 3,401,025 9/ 1968 Whitney 29/ 191.2 X 3,071,526 1/1963 Litt75/208 R X 2,855,296 10/1958 Koehring ..75/208 R 2,372,607 3/1945Schwarzkopp ..75/208 R Primary Examiner-Leland A. Sebastian AssistantExaminer-R. E. Schafer Attorney-Kinney, Alexander, Sell, Steldt &Delahunt 5'7 7 ABSTRACT 17 Claims, 5 Drawing Figures PATENTED FEB] 3I975 3 716, 347

)1 w I H 1 [/v VEN TORJ THEOO OPE E. BERGS mom PHIL /P D TA KKUNEN A rTORNE Y8 METAL PARTS JOHNED WITH SINTERED POWDERED METAL This inventionrelates to powder metallurgy. In another aspect it relates to themetalworking field in which metal parts are joined or bonded together toform a metallurgically integral metal article.

In the metalworking art it is common to fabricate articles, such asstructural members, by joining metal parts by means of metal fasteners,such as bolts and rivets, adhesives, welding, and brazing. Each of thesemeans of joining metal parts are useful but all of them havelimitations. Mechanical fasteners require the tedious placement of ahost of fasteners which significantly add to the weight and size of thefabricated article. Structural adhesives are limited by temperature andweathering conditions encountered in the use of the fabricated article.Welding and brazing have their drawbacks (see US. Pat. No. 3,496,630),requiring the use of molten metal to form a fusion bond. Also, brazingmaterials may give rise to galvanic corrosion due to the fact that theyhave a composition different than the metal parts to be joined. Weldingresults in heat-affected zones and may cause severe distortion of theparts to be joined. Though it is known (see U.S. Pat. No. 3,170,324) tobond metal parts together solely by solid state diffusion, thistechnique requires close tolerances and high pressures and/ortemperatures. Solid state diffusion with the interposition between themetal parts of a very thin dispersion of refractory powdered metal in avolatile paste has been limited to refractory metals. The majormechanism of the bond formed by this latter technique has beenpostulated as a grain-boundary migration across the contact plane of themetal parts being joined.

in the accompanying drawing,

FIGS. 1, 2, and 3 illustrate in cross-section various types of jointsformed between metal parts in accordance with this invention, namely, abutt joint, a lap joint, and a T-joint, respectively;

FIG. 4 illustrates in cross-section a cylindrical joint made accordingto this invention between a metal cylinder and a metal disc closing theend thereof; and

P10. 5 is a perspective and partial cross-section view of a honeycombstructure illustrating the application of this invention.

Briefly, according to this invention a green mixture of powdered metal(preferably a powdered non-refractory metal, such as powdered stainlesssteel) and an organic heat-fugitive binder is interposed between metalparts to be joined, the parts are assembled in the desired jointconfiguration, and the interposed mixture heated to volatilize,burn-off, or otherwise remove the binder and to sinter the powderedmetal, thereby forming a sintered metallurgically integral joint betweenthe metal parts, the bonded assembly being in effect a single piece ofmetal.

The term metallurgically integral in this context means that there is asolid state or interatomic diffusion, i.e., there is a solid state or aliquid phase sinter bond, or a combination of both types of sinter bondsbetween contiguous powdered metal particles and between the surfaces ofthe metal parts and powdered metal particles contiguous therewith. Atthe juncture between the sintered powdered metal and the metal partcontiguous therewith, there will be a solid state diffusion zone of thepowdered metal and said metal part with the balance of the sinteredpowdered metal between the adjoined metal parts having a density lessthan that of the theoretical density of the powdered metal (e.g. 40-98percent of theoretical density), a uniform microporosity, and a grainstructure free of dendritic grains. When the assembly of the metal partsand green joint are heated as a whole at sintering temperatures tosinter the powdered metal, the portion of the metal parts adjacent thesolid state diffusion zones of the joint are free of heat-affectedzones, unlike that which occurs when metal parts are joined by welding.Localized sintering, i.e., at the joint, can also be used.

Assemblies of metal parts joined by the practice of "this'invention havedesirable mechanical properties such as high tensile and fatiguestrengths and thermostability at high temperatures. The amount ofpowdered metal necessary to effect such joints is relatively small andthe weight and space occupied by the sintered powdered metal arerelatively minor. Further, close dimensional tolerances between themetal parts to be joined (normally necessary in successful brazing,welding, or diffusion bonding) are not necessary to achieve a strongjoint by the practice of this invention, though such tolerances may beobtained and retained if desired. The application of powder metallurgyin the joining of metal parts also has the advantage in that the joiningmaterial (i.e., the sintered powdered metal) can be of the samecomposition as the metal parts to be joined, thus avoiding or minimizinggalvanic corrosion and thermal fatigue. Other advantages will beapparent to those skilled in the art.

The types of metals which can be joined by the practice of thisinvention can vary, such metals including any of those commonly used forstructural purposes, particularly the transition metals of the 4th, 5th,and 6th periods of the Periodic Table (as described on p. 448-449 ofHandbook of Chemistry & Physics, 34th Ed., Chem. Rubber Pub. Co.), e.g.iron, nickel, cobalt, copper, titanium, beryllium, and chromium, andalloys thereof such as stainless steel and other steels, inconel,nichrome, and monel. Of course, the parts to be joined cannot havemelting points lower than the sintering temperatures necessary to effectthe bond made in accordance with this invention.

The metal parts to be joined can be made of wrought metal, cast metal,sintered powdered metal, or porous metal, and joints can be made ofcommon metal parts, e.g., wrought metalto-wrought metal, or differentmetal parts, e.g., wrought metal-to-sintered powdered metal. Anadvantage of joining porous metal parts in accordance with thisinvention over brazing is that the powdered metal of the bond producedby this invention does not wick or penetrate the porous metal parts, toa significant degree.

Before joining the metal parts in accordance with this invention, itfirst generally will be desirable to prepare their surface to ensure agood bond. For example the surface of the parts can be mechanicallycleaned with a wire brush or abrasive blasting and then chemicallycleaned. The surfaces of the members to be joined may be modified by theuse of surfactants, etching agents, or mechanical abrasion so as tochange the surface energy of the parts and hence the degree of theirwettability by the green material. The joining of stainless steel metalparts by the practice of this invention is a preferred embodimentthereof (particularly when stainless steel powdered metal is used toform the joint). The stainless steels are preferably chromium- 7 astypes 301 302, 304, 305, 316 and 347.

The powdered metals used in the practice of this invention, in additionto the stainless steels mentioned above, representatively include knownsinterable metals used in conventional powder metallurgy such as iron,copper, nickel, beryllium, chromium, cobalt, molybdenum, tantalum,titanium, tungsten, and alloys thereof. The stainless steel metalpowders disclosed in copending application Ser. No. 743,588, now U.S.Pat. No. 3,620,690 will be particularly useful because of the enhancedbond strength and corrosion resistance which can be obtained.Precipitation hardening stainless steel, e.g. PH-l-7 Mo, and nickelbased alloys, e.g. lnconel-625 can also be used. The non-refractorymetals and alloys being preferred as powdered metals because of thelower sintering temperatures which can be used in sintering them in thefabrication of joints. The powdered metal to be used will depend uponthe particular metal used in the parts to be joined, the desiredsintering temperature, and whether a porous or non-porous joint isdesired. Though the two metals may differ, it will be desirable if theyare both the same where differential thermal expansion or galvaniccorrosion is to be avoided.

The size of the powdered metal particles used in the practice of thisinvention can vary and generally the particular powdered metal used willhave a range of particle size. In some cases it will be desirable to useblends of two or more powdered metal products or mesh sizes. Forexample, powdered metal with sizes in the range of 150 microns to lessthan 1 micron can be used, such as less than 44 microns or a productwith about an average of 10 microns, or blends can be used. For someapplications, it may be desirable for the joint between the metal partsbe impervious to fluids, while in other applications it may be desirablefor the joint to be porous. The desired porosity or imperviousness ofthe joint can be achieved by proper selection of size or sizes of thepowdered material to be used and the amounts thereof. Generally, thelarger the particle size, the more porous the resulting sinteredpowdered metal bond.

In preparing the joining material used in this invention, (sometimesreferred to herein as the green bonding material), the powdered metal ofdesired particle size is blended with an organic heat-fugitive binder.Thermosetting materials, which cure to infusible solids upon heating,are particularly useful as the binder, since, when incorporated into thegreen bonding material of this invention, they will keep the appliedmaterial rigid when heated at temperatures sufficient to cure the same,and as such aid in keeping the assembled metal parts in their desiredjoint configuration prior to sintering. Thermosetting materials whichcan be used include those disclosed in U.S. Pat. Nos. 2,851,354,3,158,532 and in copending application Ser. No. 813,758, now abandoned.The amount of thermosetting material to be used, functionally stated,will be that sufficient to impart the above-discussed rigidity prior tosintering. Generally, this amount will be 1 to 20 weight percent of thegreen bonding material. Catalysts which catalyze the curing of suchthermosetting materials can also be included in the green bondingmaterial, including those catalysts conventionally used forthermosetting materials and those disclosed in said copendingapplication, such silver trifluoromethylsulfonate. Thermoplasticmaterials can also be used such as those disclosed in U.S. Pat. Nos.2,593,943 and 2,709,651.

Representative thermoplastics which can be used as binders includepolyolefins, polystyrenes, polyamides, acrylics, polyacetate,polycarbonates, and polyethers. Various vehicles can be used inconjunction with these binders, such as water, as well as variousplasticizers, such as glycerin, polypropylene glycol, and the like.These vehicles can be miscible or immiscible with the binder andgenerally will be used in amounts sufficient to provide the desiredviscosity to the green bonding material and control its placement orshape. The vehicle should be volatilizable below the curing temperatureof the thermosetting resin, where used. Thixotropic and wetting agentscan be incorporated into the green bonding material to control its flowcharacteristic.

The blending of the components of the green bonding material can becarried out in a conventional manipulative manner in various types ofcommercially available mixers, blenders, tumblers, and the like, carebeing taken to ensure that the blend is homogeneous and the componentswell-dispersed. The resulting blend will be in the nature of a slurry orplastic mass or dough, depending on its temperature, and depending onthe particular materials and amounts thereof used and the extent andconfiguration of the joint to be made.

The green material can be applied to the locus of the metal parts to bejoined by various means depending upon its consistency and the nature ofthe joint to be made. Where the mixture comprising powdered metal andbinder is in the nature of a plastic mass, it can be shaped on a rubbermill, calendered, or knife-coated to the desired thickness and thendried to form a green sheet or film having a leathery nature. The greensheet can then be cut to the desired shape and size. In the case of aslurry, the metal parts can be dipped into the slurry or painted withit. Other methods of application will become apparent.

The green material of this invention can be interposed between the metalparts to be joined and the combination assembled and held in the propercontiguous relation using suitable fixtures or the like where necessary.The parts are preferably pressed together sufficiently to ensure thatthe green material spreads over the desired locus of the joint to beformed. The thickness of the interposed green material can vary and canbe as much as 0.0l0 inch or greater. Where the parts to be joined lie indifferent planes, as in the case of a lap joint or T-joint, the greenmaterial will form an exposed concave fillet at the exposed extremitiesof the joint, such a fillet adding to the strength of the joint. Theassembly of the parts is then heated to volatilize the vehicle and curethe binder. The integral green structure is then sintered under vacuumor a suitable atmosphere, such as a reducing atmosphere like hydrogen ordissociated ammonia. Sintering atmosphere, temperature, and duration ofsintering will depend upon the particular powdered metals used, theselection of these conditions being within the skill of the art. In thecase of the austenitic stainless steels mentioned above, a hydrogen ordissociated ammonia atmosphere with a dew point of 40F. or lower andsintering temperatures in the range of 1,000 to 1,375C., preferablyI,200 to l,300C., will be suitable, and the duration of sintering willusually be from minutes to 2 or 3 hours.

As is evident from the above, the material joining the metal parts inaccordance with this invention is made entirely from powdered metalswithout requiring or employing wrought metal components. The joint isconstructed without welding or brazing and is, therefore, free of thedisadvantages associated with such prior art means of joining metalparts. An important advantage or feature of the joint of this inventionis the integral metallurgical or sinter bond between the adjoined metalparts.

This invention can be used to bond metal parts in various jointconfigurations or designs, such as butt joints, lap joints, anglejoints, cylindrical joints, corner joints, stiffener joints and thelike. In the accompanying drawing FIGS. 1-4 illustrate various jointdesigns which can be made. FIG. 1 illustrates a butt joint wherein metalparts 1,2 are joined by sintered powdered metal 3. FIG. 2 illustrates alap joint wherein metal parts 4,5 are joined by sintered powdered metal6, the exposed extremities of the sintered powdered metal being in theform of a fillet 7. FIG. 3 illustrates a T-joint in which the metalparts 8,9 are joined by sintered powdered metal 10, the exposedextremities of which are also in the form of a fillet 11. FIG. 4illustrates a cylindrical joint, the assembly illustrated in thisembodiment comprising metal cylindrical part 12 the end of which isclosed by a metal disc 13 bonded to the inner wall of the cylinder bysintered powdered metal 14, the exposed extremity of which again is inthe form of a fillet 15. Generally, fillets such as illustrated in FIGS.2-4 will be formed when the metal parts to be joined are in differentplanes. Such a fillet increases the strength of the bond between themetal parts and prevents stress risers. In order to form such a fillet,the heat-fugitive binder mixed with the powdered metal will preferablycomprise a thermosetting material as described, such as an epoxide, andthe green mixture will be of sufficient consistency or viscosity suchthat after placement the shape of the fillet can be controlled. Whenheated, the thermosetting material will cure in the form of a solid andwill be vaporized or burned-off as a solid, or otherwise removed fromthe joint.

FIG. 5 illustrates a honeycomb sandwich or panel which can be fabricatedin accordance with this inven tion, such panels being useful asstructural members in aircraft or the like. This panel can comprise, as

wrought metal parts, a base sheet 16, a face sheet 17, and a honeycombstructure 18 disposed therebetween. The tops and/or bottoms of the websmaking up the honeycomb structure 18 are bonded to said sheets 16,17 bysintered powdered metal 19, this type ofjoint being shown in detail inFIG. 3. The fillet shown there, in a honeycomb structure, is an exampleof formation of a fillet by the practice of this invention which isotherwise unobtainable as a practical manner by conventional welding.Unlike fabrication of honeycomb structures bonded in accordance withthis invention, thin honeycomb webs tend to deform and melt duringwelding. The honeycomb sandwich of FIG. 5 can be prepared by dipping thetops and. bottoms of the honeycomb structure 18 to a sufficient depth ina slurry of powdered metal and heat-fugitive binder so as to applythereto a sufficient amount of the slurry necessa ry to effect a bondwhen such a honeycomb structure is sandwiched between sheets 16,17 andthe assembly heated as described above.

A slurry (or slip) particularly useful for forming a joint betweenwrought metal parts of a honeycomb structure such as illustrated in FIG.5 can be made up of weight percent powdered stainless steel AISI Type304 having an average particle size of 10 microns, 14.8 weight percentof polypropylene glycol vehicle (with a molecular weight of about 400),5 weight percent of Epon-IOOI epoxide, 0.025 weight percent of silvertrifluoromethylsulfonate, and 0.2 weight percent of Calidria asbestos asa thixotropic agent. This slurry can be made by adding the powderedmetal to a mixture of the other components, which mixture can have aconsistency of l,l00 to 1,400 cps. Honeycomb, made of Type 347 stainlesssteel having inch sq. cells, 0.002 inch web thickness, and k inch inheight, is dipped into said slurry so as to deposit on the bottoms ofthe webs a sufficient amount of the slurry. The dipped honeycomb is thenlayed on an impervious backing sheet of Type 347 stainless steel havinga thickness of 0.018 inch and lightly pressed to ensure good contact andfillet formation. The assembly is heated for 2 hours at 150F. to lowerthe viscosity of the bonding material and enhance settling of thepowdered metal. The temperature is then raised to 350F. and held therefor 2 hours to volatilize the polypropylene glycol. The assembly is thenheated at 400F. for 1 hour causing the curing of the epoxide to form athermoset resin. At this point, rigid fillets are present between themetal parts of the assembly, these fillets comprising about weightpercent of powdered stainless steel. The top or opposite side of thehoneycomb is assembled in a similar manner with a perforated sheet ofstainless steel (Type 347) having a thickness of 0.018 inch and 0.050inch holes on 0.093 inch centers. The assembly is then heated to aboutl,200C. in hydrogen to remove the epoxide and to cause the powderedmetal to sinter and provide the sinter joints of this invention asdescribed above. As modification, the top or face sheet of the honeycombstructure can be used in non-perforated form. In this lattermodification, where the backing sheet and facing sheet are bothnon-perforated, it will be necessary to provide for escape of volatileand gaseous products during the heating of the assembly. This can beaccomplished by using honeycomb which has holes in its web.

As another modification, the perforated face sheet can have a porouspowdered metal sheet sintered to the top surface thereof as disclosed incopending application Ser. No. 27,079 filed on even date herewith, nowU.S. Pat. No. 3,693,750 such a honeycomb structure being useful as anacoustic panel.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art from the foregoing descriptionwithout departing from the scope and spirit of this invention.

' What is claimed is:

1. An article comprising wrought, cast, previously sintered, or porousmetal parts joined together by a metallurgically integral joint ofsintered powdered metal interposed between contiguous surfaces of saidmetal parts.

2. The article of claim 1 wherein said metal parts and powdered metalare stainless steel.

3. The article of claim 1 wherein said article is a honeycomb sandwichstructure made of wrought metal parts.

4. The article of claim 1 wherein said joint is in the form of a fillet.

5. The article of claim 1 wherein said joint is a T- joint.

6. The article of claim 1 wherein said joint is a lap joint.

7. A method for forming an article according to claim 1, which comprisesinterposing between said metal parts a mixture comprising powdered metaland a heat-fugitive binder, assembling said parts in a desired jointconfiguration, and heating the assembly to remove said binder and sintersaid powdered metal to form said metallurgically integral joint.

8. The method according to claim 7, wherein said binder comprises athermosetting material.

9-. An article comprising wrought, cast, previously sintered, or porousmetal parts joined together by a green bonding material comprising amixture of powdered metal and a solid heat-fugitive, cured,thermosetting material as a binder.

10. A honeycomb sandwiched structure made of wrought metal partscomprising a metal honeycomb structure disposed between two metalsheets, and sintered powdered metal in the form of metallurgicallyintegral fillet joints joining the contiguous portions of said honeycombstructure and said sheets.

11. The structure according to claim 10 wherein said two metal sheetsare non-perforated and wherein said honeycomb structure has holes in itsweb.

12. The structure according to claim 10 wherein one of said sheets is animpervious backing sheet and the other sheet is a perforated face sheet.

13. The structure according to claim 12 wherein a porous metal sheet issintered to the top surface of said perforated face sheet.

14. The structure according to claim 13 wherein said porous metal sheetis a sintered powdered metal sheet.

15. A method for forming a honeycomb sandwiched structure made ofwrought metal parts comprising a previously metal honeycomb structureinterposed between two metal sheets, which comprises interposing betweensaid honeycomb structure and said sheets a mixture comprising powderedmetal and a heat-fugitive thermosettingf binder, and heating theassembly to remove said lnder and sinter said powdered metal to formmetallurgically integral joints between contiguous portions of saidhoneycomb structure and said sheets.

16. The method according to claim 15 wherein one of said sheets is animpervious backing sheet and the other sheet is a perforated facingsheet having a porous sintered powdered metal sheet sintered to the topsurface thereof.

17. The article of claim 1, wherein said metal parts are wrought metalparts.

v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,71,3 Dated February 13, 1913 Inventor) I Theodor-e R. Bergstrom & PhilipD. Talckunen It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shcwn below:

Column 7, line n, "27,079"' h read -"r3 -890 Signed and "ee'a led this17th day cf December 1974.

(SEAL) Attest:

McCOY M. GIBSON JR; c. MARSHALL DAM: Attest ing Officer Commissioner(fif -Patents FORM PO-IOSO (10-69) (SEAL) Attest:

MCCOY M. GIBSON JR. 'c, MARSHALL NN Commissioner" Cat-Patents UNITEDSTATES PATENT OFFICE a CERTIFICATE OF CORRECTION Patent Na. 3,716,3 1?Dated Februarv 12. 19?? t Theodore R. Bergstr'om & Philip D. Takkunen Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as ehewn below:

Column 7, line n, "27,o79"-s u1d read "73,3 9 0 7 s gned arid sealedthis 17th da of Decemlier 1974.

Attesting Officer.

1. An article comprising wrought, cast, previously sintered, or porousmetal parts joined together by a metallurgically integral joint ofsintered powdered metal interposed between contiguous surfaces of saidmetal parts.
 2. The article of claim 1 wherein said metal parts andpowdered metal are stainless steel.
 3. The article of claim 1 whereinsaid article is a honeycomb sandwich structure made of wrought metalparts.
 4. The article of claim 1 wherein said joint is in the form of afillet.
 5. The article of claim 1 wherein said joint is a T-joint. 6.The article of claim 1 wherein said joint is a lap joint.
 7. A methodfor forming an article according to claim 1, which comprises interposingbetween said metal parts a mixture comprising powdered metal and aheat-fugitive binder, assembling said parts in a desired jointconfiguration, and heating the assembly to remove said binder and sintersaid powdered metal to form said metallurgically integral joint.
 8. Themethod according to claim 7, wherein said binder comprises athermosetting material.
 9. An article comprising wrought, cast,previously sintered, or porous metal parts joined together by a greenbonding material comprising a mixture of powdered metal and a solidheat-fugitive, cured, thermosetting material as a binder.
 10. Ahoneycomb sandwiched structure made of wrought metal parts comprising ametal honeycomb structure disposed between two metal sheets, andsintered powdered metal in the form of metallurgically integral filletjoints joining the contiguous portions of said honeycomb structure andsaid sheets.
 11. The structure according to claim 10 wherein said twometal sheets are non-perforated and wherein said honeycomb structure hasholes in its web.
 12. The structure according to claim 10 wherein one ofsaid sheets is an impervious backing sheet and the other sheet is aperforated face sheet.
 13. The structure according to claim 12 wherein aporous metal sheet is sintered to the top surface of said perforatedface sheet.
 14. The structure according to claim 13 wherein said porousmetal sheet is a sintered powdered metal sheet.
 15. A method for forminga honeycomb sandwiched structure made of wrought metal parts comprisinga previously metal honeycomb structure interposed between two metalsheets, which comprises interposing between said honeycomb structure andsaid sheets a mixture comprising powdered metal and a heat-fugitivethermosetting binder, and heating the assembly to remove said binder andsinter said powdered metal to form metallurgically integral jointsbetween contiguous portions of said honeycomb structure and said sheets.16. The method according to claim 15 wherein one of said sheets is animpervious backing sheet and the other sheet is a perforated facingsheet having a porous sintered powdered metal sheet sintered to the topsurface thereof.